Method of forming a pinhole-free alloy layer on the surface of a base made of aluminum or an aluminum alloy

ABSTRACT

A method of forming a pinhole-free alloy layer on the surface of a base made of aluminum or an aluminum alloy, said method being characterized by applying a thin film of hydrocarbon on the surface of the base, spraying and depositing fine particles of a metal halide over said film, heating the coated base in a furnace to over the temperature at which a sublimation of aluminum halide takes place and then cooling it.

United States Patent 1111 3, 9,

[ 72] Inventors Tadashi Nakano 56] References Cited 9 8 UNITED STATES PATENTS Keuchi Henrm, l-l2-9, Haruyoshi, both of 242 649 6 H881 owes 1 17/22 [21] A 1 N0 2:35 2,161,597 6/1939 Swartz.... 117/22 [22] M 29 1968 2,512,455 6/1950 Alexander 1 117/22 a 3,022,193 2/1962 Goodzeitet a1 117/50 [45] Patented Nov. 9, 1971 Primary Examiner-Alfred L. Leavitt Assistant Examiner-Janyce A. Bell An0rneyHo1combe, Wetherill and Brisebois [54] METHOD OF FORMING A PlNHOLE-FREE ALLOY LAYER ON THE SURFACE OF A BASE MADE F ALUMINUM 0R AN ALUMINUM ALLOY 0 ABSTRACT: A method of formmg a p1nhole-free alloy layer 4 Cl 3 D on the surface ofa base made of alummum or an alummum a1- 10y, said method being characterized by applying a thin film of [52] U.S. Cl 117/22, hydrocarbon 0n the surface of the base, spraying and deposit- 117/33, 117/50, 117/130 R, 117/46 CA ing fine particles of a metal halide over said film, heating the [51] Int. Cl C23c 17/02 coated base in a furnace to over the temperature at which a [50] Field of Search 1 17/50, 22, sublimation of aluminum halide takes place and then cooling 46CA,33,130R,131,22

msmcum 9 Ian 3.619.232

ll ll ll /llllllll 'IIIIIIIIIIIIIIIII III/I11 K" '5 METHOD OF FORMING A PlNHOLE-FREE ALLOY LAYER ON THE SURFACE OF A BASE MADE OF ALUMINUM R AN ALUMINUMALLOY DISCLOSURE This invention relates to a method of forming a pinhole-free alloy layer on the surface of aluminum or an aluminum alloy base, said method being characterized by three steps, the first being to apply a thin film of an adhesive hydrocarbon on the surface of said base, the second step being to spray and deposit fine particles of a metal halide over the surface which has undergone the first step, and the third step being to put the coated base into a furnace, where it is heated to over the temperature at which a sublimation of aluminum halide takes place, and then to cool it.

The only conventional method available of solidly depositing a dissimilar metal on the surface of an an aluminum or an aluminum alloy base has been plating, but even now the plating is not successful because of pinholes inevitably developing in the dissimilar metal layer and causing local cells. The following phenomena may be mentioned as possible causes of pinhole formation: The oxide film covering the surface of base metal inhibits the reaction between base metal and molten metal salt; under the pressure of the molten metal salt there is no escape for the aluminum halide generated by reaction between the base metal and molten metal salt; the surface of the base metal is attacked by the hydrohalic acid formed through contact between a wet base metal and the molten metal salt, and so on.

The present invention provides a method, preventing pinholes in the formation of a dissimilar metal alloy layer on the surface of aluminum or an aluminum alloy as base metal, making the formed alloy layer penetrate deep into the base metal, thereby eliminating the occurrence of the local cell phenomenon, and forming the above-mentioned alloy layer in a simplified manner on whatever surface, locally or wholly, be it flat, curved or irregular.

According to this invention, in the first step the aluminum or aluminum alloy surface is thinly coated with an adhesive hydrocarbon and in the second step it is sprayed with fine particles of halide salts of such metals as copper, zinc, tin, etc. Thereby these fine particles cover the metal surface, locally or wholly, and with a uniform thin layer, be it a flat or curved or irregular surface. Moreover, in the third step until the metal is heated in the furnace, the fine particles are not displaced to one side; accordingly the operation is made easier and the formed alloy layer becomes even. And by applying an adhesive hydrocarbon locally or wholly on the base metal surface, the alloy layer can be deposited locally or wholly on the surface. Meanwhile, in the room-temperature operation outside the furnace the film of adhesive hydrocarbon separates the particles of metal halide from the surface of base metal; therefore even when said particles of metal halide absorb the atmospheric moisture during operation, there is no likelihood of the metal surface being corroded. Thus, any corrosion of the metal surface at room temperature before the start of the third step, which is the cause of pinhole formation in the third step can be prevented.

Further in the third step, the adhesive hydrocarbon which vanishes at low temperature is decomposed and vanishes when the heating in the furnace is started, but before it vanishes, it acts, together with the metal halide, as a reducing agent, thereby contributing to the removal of the oxide film from the surface of base metal as well as to the penetration of the component metal of the metal halide into the base metal. Also in the third step when the metal is heated to over the temperature for sublimation of aluminum halide in the furnace and the hydrocarbon vanishes through decomposition, both the metal halide and the base metal are heated to a molten state but thereby, since the base metal has by far larger thermal capacity than the fine particles of the metal halogenide salt and the former has high heat conductivity, the fine particles of the metal halogenide salt slowly turn into a molten state beginning from one part of the metal surface and steadily extending over 'the entire surface. Accordingly the two reactions: between the molten metal halide and the base metal, i.e., halogen vs. base metal, and between the component metal of the'metal halide and the base metal, will not take place simultaneously over the entire surface of the base metal, but they will start from the first molten part and spread over the entire surface with an extension of the molten state. Therefore, the aluminum halide formed through the reaction of halogen vs. base metal will be heated to sublimation at the time that it forms and will be easily taken out of the reaction system without having any effect on the formed alloy layer; thus, an alloy layer free from pinholes can be formed with deep penetration into the surface of the base metal of aluminum or aluminum alloy layer.

The embodiment of this invention will be described with reference to the attached drawings, in which:

FIG. 1 is an enlarged section showing the state following the second step of this invention;

FIG. 2 shows an enlarged section of a'product of this invention and FIG. 3 shows an enlarged section of a conventional aluminum product, the surface of which has been deposited with a'dissimilar metal.

As shown in FIG. 1., the base metal 1 is aluminum or an aluminum alloy. Over the surface of the base metal 1, any adhesive hydrocarbon, say, petrolatum 2 is evenly spread to a small thickness; and on this petrolatum-coated surface, crystal particles 3 of a metal halide salt, say, cuprous chloride, are spread. Then the excessive portion of the fine crystal particles 3 of cuprous chloride is removed by tilting the surface of the base metal 1 or by blasting with air; next, the base metal 1 is inserted into a furnace to be heated or inserted into a furnace already heated to a considerably high temperature; thereupon before the petrolatum 2 vanishes through volatile decomposition, the fine crystal particles of cuprous chloride 3 will be dried and no chemical reaction will occur between the base metal I and these particles coming into direct contact with the base metal 1.

As the base metal 1 continues to be heated in the furnace and the temperature reaches about 450 C., first the particles 3 of cuprous chloride will begin to melt on one part of the surface of the base metal 1 and the reaction will take 'place between the molten salt and the surface of the base metal 1, Le, halogen vs. base metal 1 and copper vs. base metal 1. This will be followed by a fuming sublimation of aluminum chloride, which will steadily cover the whole surface of the hydrocarbon film.

The cessation of the aluminum chloride fume signals the conclusion of the sublimation and thereupon the heating is immediately stopped to let the base metal 1 cool down to the room temperature. Then, after cooling, the residue on the surface is flushed away with water, the target metal product 4 will be obtained, that is, the metal surface is found to be deposited with a pinhole-free copper layer which represents an alloy of copper and base metal, said layer gradually thinning out into the core of base metal. The adhesive hydrocarbon referred to in FIG. 2 may be liquid paraffin, petrolatum, or any hydrocarbon which decomposes and vanishes at the temperatures employed. The metal halide salt may be cuprous chloride, cupric chloride, zinc chloride, tin chloride, etc. will be available; and the resulting effect will be nearly the same as in this embodiment, whatever of them may be adopted.

FIG. 3 shows a section of a product according to a conventional plating method and a dissimilar metal layer 5 is plated on the base metal 1' but pinholes 6 are formed therein.

This invention may be embodied in other specific forms without departing from the spirit thereof. The embodiment disclosed is therefore to be considered as illustrative rather than restrictive, the scope of the invention being indicated by the appended claims.

What is claimed is:

l A method of forming a pinhole-free alloy layer on a base having a surface of aluminum or an aluminum alloy which method comprises the steps of:

applying to said surface a thin film of an adhesive hydrocarbon which decomposes and vanishes through volatile decomposition leaving no residue at the temperatures to which said base is subsequently subjected to prevent corrosion of said base by the metal halide during the metal halide coating step and prior to the heating step;

spreading fine particles of a metal halide over said adhesive hydrocarbon film;

heating said base to a temperature high enough to sublime aluminum halide and melt said metal halide particles, during said heating step said decomposing hydrocarbon and said metal halide act as reducing agents which assist in removing oxide film from said base, and then cooling said base. 2. A method of claim 1, wherein said metal halide salt is selected from the group consisting of cuprous chloride, cupric selected from the group consisting of cuprous chloride, cupric chloride zinc chloride and tin chloride.

* f t i i 

2. A method of claim 1, wherein said metal halide salt is selected from the group consisting of cuprous chloride, cupric chloride, zinc chloride and tin chloride.
 3. The method claimed in claim 1 in which said hydrocarbon is paraffin.
 4. The method claimed in claim 3 in which said halide is selected from the group consisting of cuprous chloride, cupric chloride, zinc chloride and tin chloride. 